The invention disclosed herein relates generally to backlighting for display panels which are at least partially translucent, and to corresponding illuminated display devices of the type incorporating backlighting from a high aspect ratio illumination source. More particularly the invention relates to such a display device and illumination source employing light emitting diodes in conjunction with an optical cavity and enhancement films to provide improved viewability from a relatively wide range of viewing angles.
There are various situations in which it is desired or necessary that information be displayed so as to be readable under a wide range of ambient light conditions and throughout a relatively wide range of viewing angles. In many of these situations it is also highly desirable, if not required, that the display apparatus be capable of automatic updating of the displayed information and/or permitting selection of the information to be displayed. Another characteristic which is desired and often important is that the apparatus be robust and require minimum maintenance.
A specific application requiring such a display device relates to monitoring the status and/or operation of environmental control equipment on mobile shipping containers for perishable goods. Such containers, which each have a refrigeration unit and/or other environmental control apparatus, may, during a single trip, be moved by train, track and/or marine vessel, and moved from one mode of transportation to another by crane or other cargo handling equipment. During the shipping process, it may be necessary to periodically check on operation of the refrigeration or other environmental control equipment and the status of environmental conditions within the container. A display device for presenting this information is typically installed at a convenient location on each container. Since it may be necessary to perform a status check at any time of the day or night and under varying climatic conditions, the display must be illuminated so as to be easily readable during daylight as well as dark conditions. Further, since the containers may be stacked as many as three high, and on platforms such as a truck or rail car bed, it may be necessary to read the display from a variety of viewing angles ranging between substantially above to somewhat below horizontal, and within a reasonable range of horizontal viewing angles.
The requirements for long life, robustness and minimum maintenance essentially dictate solid state design and construction. Thinness or high aspect ratio of the display apparatus is also an important consideration in many applications, such as the previously described shipping container environmental control and status monitoring system.
Although several flat panel technologies are available for producing illumination, such technologies including solid state florescent and electroluminescent devices, light sources based on these technologies have various disadvantages, such as high initial cost, excessive degradation of light output with time, limited useful life and/or requirements for complex driver or energization circuitry.
Light emitting diodes which produce a relatively high light output are readily available, and inexpensive, and have modest energization circuitry requirements. However, such diodes are essentially point light sources which, if used to backlight a relatively large area, must be combined with other optical elements for evenly distributing and directing the light output.
A variety of multifaceted lens products are now available for achieving a large aperture with a thin element. One such family of products includes transmissive and reflective optical films available through Minnesota Mining and Manufacturing Company, and described in U.S. Pat. Nos. 4,799,137, 4,874,228, 4,984,144, 5,054,885 and 5,190,370. These films are characterized by a major surface formed with an array regularly spaced, parallel, fine, triangular prisms which serve to redirect light received from a range of angles to a relatively uniform direction normal to the nominal plane of the film. This characteristic can be utilized to implement a thin, large aperture light source by projecting light produced by one or more point sources onto the film at a very small angle relative to the nominal plane of the film.
One of the more difficult design problems with such a light source implementation is the selection and arrangement of materials which will achieve illumination of uniform intensity and direction over the entire aperture. Uniform intensity and direction of illumination is required to avoid "hot spots", voids, streaks, an inadequate range of angles for acceptable viewing and other defects.
The applicant has discovered a unique combination and arrangement of optical elements which provides for enhanced viewing of a display backlit with a thin, large aperture light source, thereby avoiding many of the disadvantages of prior known display devices.